1. Field of the Invention
The present invention relates to a gas control apparatus that performs gas control.
2. Description of the Related Art
Various types of gas control apparatuses that perform gas control by using a plurality of pumps have been devised. For example, Japanese Unexamined Utility Model Registration Application Publication No. 62-52291 discloses a two-stage vacuum pump capable of switching a state of connection between two pumps between a series connection and a parallel connection depending on the situation.
FIG. 17A is a diagram illustrating the flow of air in a two-stage vacuum pump 1 according to Japanese Unexamined Utility Model Registration Application Publication No. 62-52291 in the state in which a first-stage pump 4 and a second-stage pump 5 are connected in parallel. FIG. 17B is a diagram illustrating the flow of air in the two-stage vacuum pump 1 according to Japanese Unexamined Utility Model Registration Application Publication No. 62-52291 in the state in which the first-stage pump 4 and the second-stage pump 5 are connected in series. FIG. 18 is a sectional view of a directional control valve 9 illustrated in FIGS. 17A and 17B.
As illustrated in FIGS. 17A and 17B, the two-stage vacuum pump 1 includes a pump suction hole 2, a pump ejection hole 3, the first-stage pump 4, the second-stage pump 5, a check valve 7, and the directional control valve 9.
The inlet of the first-stage pump 4 is connected to the pump suction hole 2 and the check valve 7, and the outlet of the first-stage pump 4 is connected to an introduction hole 912 of the directional control valve 9. The inlet of the second-stage pump 5 is connected to a discharge hole 913 of the directional control valve 9, and is also connected to the pump suction hole 2 with the check valve 7 provided therebetween. The outlet of the second-stage pump 5 is connected to another discharge hole 914 of the directional control valve 9, and is also connected to the pump ejection hole 3. The check valve 7 allows air to flow from the pump suction hole 2 to the second-stage pump 5, but does not allow air to flow from the second-stage pump 5 to the pump suction hole 2.
As illustrated in FIG. 18, the directional control valve 9 includes a columnar casing 921 and a valve body 925 disposed in the casing 921. The casing 921 has the introduction hole 912 and discharge holes 913 and 914, valve chests 922 and 923, an introduction path 924 that connects the valve chests 922 and 923 to each other, and ring-shaped elastic members 932 and 934. The valve body 925 is accommodated in the valve chests 922 and 923 and the introduction path 924 such that the valve body 925 is slidable in an axial direction of the casing 921. The valve body 925 includes a rod 926 and disc-shaped valve portions 927 and 928 provided at the ends of the rod 926.
In the above-described structure, when a pressure difference between the pump suction hole 2 and the pump ejection hole 3 is smaller than or equal to a predetermined value, the valve body 925 of the directional control valve 9 slides toward the valve chest 922. Thus, a peripheral portion of the valve portion 927 comes into contact with the ring-shaped elastic member 934, so that the introduction hole 912 and the discharge hole 914 communicate with each other. As a result, the first-stage pump 4 and the second-stage pump 5 are connected in parallel as illustrated in FIG. 17A.
When the pressure difference between the pump suction hole 2 and the pump ejection hole 3 becomes greater than or equal to the predetermined value, the valve body 925 of the directional control valve 9 compresses a spring 929 and slides toward the valve chest 923. Thus, a peripheral portion of the valve portion 928 comes into contact with the ring-shaped elastic member 932, so that the introduction hole 912 and the discharge hole 913 communicate with each other. In addition, the check valve 7 is closed. As a result, the first-stage pump 4 and the second-stage pump 5 are connected in series as illustrated in FIG. 17B.
In the above-described valve structure of the directional control valve 9, to prevent air leakage through a gap between the valve body 925 and an inner wall of the casing 921 and allow the valve body 925 to slide smoothly, grease is generally applied between the valve body 925 and the inner wall of the casing 921. However, this method has a problem in that small particles of the grease (oil mist) are mixed into the air when the air flows through the casing 921, and clean air cannot be ejected from the pump ejection hole 3.
In addition, in the two-stage vacuum pump 1, the directional control valve 9 has a large and complex structure in which the valve body 925 is disposed in a slidable manner. Therefore, it is difficult to reduce the size, in particular, the height, of the two-stage vacuum pump 1. Furthermore, the performance (flow rate and pressure) of the pump decreases as the size of the two-stage vacuum pump 1 decreases. Therefore, there is a limit to the extent to which the size of the two-stage vacuum pump 1 having the structure according to the related art can be reduced while the performance of the pump is maintained.